IBM System/360 Model 75

The IBM System/360 Model 75 is a discontinued high end / high performance[1] system that was introduced April 22, 1965.[2] Although it played many roles in IBM's System/360 lineup,[3] it accounted for a small fraction of a percent of the 360 systems sold.[4] Five Model 75 computers housed at NASA's Real Time Computer Complex were used during the Apollo program.[5]

IBM System/360 Model 75
Release dateApril 22, 1965
SuccessorModel 195


Three models, the H75, I75 and J75, were respectively configured with one, two or four IBM 2365 Model 3 Processor Storage units, each of which provided 262,144 (256K) bytes of core memory, so that the H75 had 262,144 (256K) bytes of core, the I75 had 524,288 (512K), and the J75 1,048,576 (1 MB).


The high performance of the Model 75 was attributed[1] to half a dozen advanced features, including Parallel arithmetic, Overlapped memory fetch and Parallel addition for address calculation.

Furthermore, independent storage sections provided two-way (H75) or four-way (I75, J75) interleaving of memory access. Even with only two-way interleaving, "an effective sequential access rate of 400 nanoseconds per double word (eight bytes) is possible."


The Model 75 implements the complete System/360 "universal instruction set" architecture, including floating-point, decimal, and character operations as standard features.


  1. "IBM System/360 Model 75 Functional Characteristics" (PDF). IBM. A22-6889-0.
  2. IBM Archives: System/360 Model 75
  3. high end Main processor under ASP when the latter was introduced in 1967, high end Support Processor, and more than one Time-sharing option: TSO and CALL/OS
  4. An ADP Newsletter cited on page 56 in Weiss, Eric A. (ed) (1969). Computer USage Essentials. McGraw-Hill. LCCN 71-76142.CS1 maint: extra text: authors list (link) says (re S/360) "75-91 .. 0.1%"
  5. "IBM 100 Icons of progress: The Apollo Missions". IBM. Retrieved 2019-03-26.
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